1. Field of the Invention
The present invention relates in general to a lithographic apparatus and a method of manufacture of a device using lithographic apparatus. More specifically, the invention relates to a scatterometry method measuring a back focal plane diffraction intensity image and a measurement system.
2. Background Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) of a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto a target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Angular-resolved scatterometry for critical dimension (CD) metrology measures the back focal plane diffraction intensity as obtained from dedicated test gratings. Typically multi-parameter shape-profiles of trapezoidal gratings lines are retrieved from the diffraction intensities. Examples of this measurement technique operate at best at a diffraction limit of around 70 nm for the width of a single line (0.25/NA) for UV-light with=280 nm and NA˜1. This implies that on-product structures and structured patterns that are typical for hot-spot areas generate intensities only in the 0-th order of diffraction, which is detected within the aperture. These are the so-called propagating waves. Higher-order diffraction intensities only exist within the medium, but are not detectable (and are called the evanescent waves). Still, the 0-th order beam may carry enough information such that by using a-priori knowledge of what structure has been written on the wafer, but the parameters of such a high-frequency structure may be retrieved only up to a certain extent, in particular up to a certain accuracy.
Moreover, information about structures that is beyond the diffraction limit is intrinsically more difficult to be retrieved, and parameters of the diffracting pattern may be highly correlated such that unambiguous and robust reconstruction may be hampered, just because of the fact that the information about the diffracting structure that is present in the back focal plane intensities is just too sparse.